The present invention is related to mechanical pencils. More particularly, it is related to mechanical carpenters' pencils with means for sharpening the working end of the pencil lead.
The vast majority of pencils are substantially round in cross section. They are available as cylindrical graphite leads encased in wooden bodies and as mechanical pencils with loose graphite leads gripped mechanically in reusable bodies. Wooden pencils are sharpened by trimming the wooden body and the lead in order to form a sharp point on the lead, thereby enabling the user to mark with precision and accuracy. Most mechanical pencils utilize small diameter leads that may be advanced as necessary and used without trimming or pointing.
Carpenters' pencils generally have substantially rectangular cross sections, a shape which has proven to be more durable than a circular cross section when marking a material having a rough surface, such as lumber. However, the noncircular cross section complicates the chore of sharpening, which is necessary for accuracy. Carpenters usually use a pocket knife or a utility knife to sharpen wood-cased pencils. A usable point can be produced in this manner, but the time required to produce a precision point is excessive.
U.S. Pat. No. 5,044,805, incorporated herein by reference, discloses a mechanical carpenter's pencil wherein a flat lead is gripped in a plastic body. No means are provided for sharpening the lead. General purpose mechanical pencils designed for writing on relatively smooth materials such as paper use thin leads having circular cross sections. Thin leads do not need to be sharpened; they can simply be advanced to provide a fresh point as the lead is consumed. On the other hand, the flat lead in a carpenter's pencil must be sufficiently thick to provide mechanical strength for the more demanding application. Advancing the thick lead without sharpening results in a blunt end, not suitable for precision marking.